The present invention relates to a stepping motor.
According to a conventional stepping motor, rotation preventing torque (hereunder sometimes referred to as detent torque) occurs owing to a structure thereof, in which magnetic pole teeth of a yoke are respectively placed corresponding to armature windings of a stator. The conventional stepping motor has an advantageous in that an object to be driven can be held at a predetermined place by effectively utilizing this detent torque. However, for example, when a microstep driving operation is performed, it may be difficult to achieve high-speed positioning of a rotor under the influence of the detent torque.
That is, the conventional stepping motor is constructed so that a rotating magnetic field is generated by winding a coil around each of the magnetic pole teeth of the yoke, which is placed in such a way as to be coaxially placed with a rotor, and applying a pulse voltage to each of the coils, and that thus, the rotor is driven to rotate by a predetermined stepping angle. Thus, a stepping angle for the rotation of the rotor is determined according to both the number of stator poles and that of rotor poles. Further, to perform a driving operation of rotating the rotor in units of a finer stepping angle, the number of the rotor poles and the number of yoke teeth are increased. Alternatively, an energizing method is devised.
Furthermore, even during a non-energized time, an attracting force acts between multi-polarized permanent magnet and each of the magnetic pole teeth, so that detent torque occurs. Thus, the conventional stepping motor has a function of self-holding a rotation angle position of the rotor.
A holding force of the rotor due to this detent torque enables the self-holding of the rotation angle position thereof during the non-energized time, whereas this holding force sometimes becomes an obstacle to an operation of driving the rotor to rotate.
Meanwhile, an object to be driven by a stepping motor is, for instance, an optical pickup disclosed in JP-A-2000-251270 and JP-A-2000-293864. This optical pickup is driven at a high speed in a radial direction of a recording/reproduction surface of an optical disk adapted to rotate at a high speed. Further, this optical pickup is required to instantaneously stop. Thus, rapid response of the stepping motor is demanded. However, sometimes, the detent torque is an obstacle to enhancement of this rapidity of the response. Meanwhile, according to a stepping motor for use in an autofocus mechanism of an optical camera, a micro-step driving operation is performed. Thus, it is requested to perform a driving operation of rotating the rotor in units of a stepping angle, which is finer than the conventionally employed stepping angle.
As described above, a stepping motor enabled to achieve a driving operation unaffected by the detent torque is desired.
Accordingly, the invention is accomplished in view of the aforementioned problems. An object of the invention is to provide a stepping motor that is prevented from being affected by detent torque during a driving operation and during a stop time, thereby to facilitate a micro-step driving operation and to improve a starting characteristic.
Further, another object of the invention is to provide a stepping motor enabled to arbitrarily set a point of generation of detent torque so as to stop a rotor at a given angular position.
Moreover, another object of the invention is to provide a stepping motor that can be constructed in such a way as to be compact and flat in a longitudinal direction along a shaft of rotation of a rotor.
In order to solve the aforesaid object, the invention is characterized by having the following arrangement.
(1) A stepping motor enabled to prevent generation of detent torque therein and arbitrarily set the detent torque, the stepping motor comprising:
a rotor including a shaft and a multi-polarized permanent magnet fixed to the shaft;
a stator including a plurality of air-core coils, each of which is made from a predetermined number turns of a wire wound around an axis perpendicular to a central axis of rotation of the rotor and is placed along a peripheral surface of the permanent magnet and face each other.
(2) The stepping motor according to (1), wherein
the plurality of air-core coils are provided as sets of first air-core coils and second air-core coils,
the set of first air-core coils faces the permanent magnet and is disposed at a circumferentially equally spaced position,
the set of second air-core coils is disposed at a circumferentially equally spaced position that is angularly spaced from the set of first air-core coils by an angular deviation, which is a phase angle of 90 degrees, and
generation of rotating torque including a couple of forces is enabled by energizing the set of first air-core coils and the set of second air-core coils.
(3) The stepping motor according to (1), wherein
the permanent magnet is polarized so that the number of poles thereof is one of 2, 4, 6, 8, 10, and 12, and
a 2-phase excitation driving operation is enabled by setting the number of the air-core coils to be one of 2, 4, 6, and 8 and evenly allocating the air-core coils to first and second phase coils, respectively, along the peripheral surface of the permanent magnet to obtain an electrical angle of 90 degrees as the angular deviation.
(4) The stepping motor according to (1), wherein each of the air-core coils is wound at an angular position, at which a center of a nearly rectilinear constituent part thereof in a direction of rotation of the rotor contributing to rotation of the rotor coincides with a corresponding one of N-poles and S-poles of the permanent magnet.
(5) The stepping motor according to (1), wherein a length of a conductor perpendicular to a magnetic field is increased as much as possible by setting a length of the nearly rectilinear constituent part of the air-core coils in a direction of rotation of the rotor, which contributes to rotation of the rotor, to be equal to or longer than a length in a longitudinal direction along an axis of rotation of the permanent magnet.
(6) The stepping motor according to (1) further comprising a supplemental yoke for arbitrarily setting a point of generation of detent torque so as to stop the rotor at a given angular, position.
(7) The stepping motor according to (1), wherein
the stator includes a resin member integrally forming arm parts radially extended from an inside diameter side to an outside diameter side on a circle surface concentric with a center of rotation of the rotor, and
the resin member holds the air-core coils so that angular position relation between the air-core coils and magnetic poles of the permanent magnet is maintained.
(8) The stepping motor according to (7), wherein a supplemental yoke for arbitrarily setting a point of generation of detent torque so as to stop the rotor at a given angular position is embedded in the resin member.
(9) A 2-phase excitation type PM stepping motor enabled to prevent generation of detent torque therein and arbitrarily set the detent torque, the stepping motor, comprising:
a rotor including a shaft and a permanent magnet fixed to the shaft, the permanent magnet being polarized so as to have poles of the number that is one of 6, 8, 10, and 12;
a stator including a total of 4 air-core coils, each of which is made from a predetermined number of turns of a wire wound around an axis perpendicular to a central axis of rotation of said rotor, and is placed along a peripheral surface of the permanent magnet and disposed by dividing the air-core coils into two sets of two air-core coils so as to face each other, wherein the two sets of two air-core coils are evenly allocated to first and second phase coils, respectively, along the peripheral surface of the permanent magnet so as to have an electrical angle of 90 degrees as an angular deviation therebetween to enable a 2-phase excitation driving operation; and
a main part serving as a yoke having flat parts obtained by planarizing a space part between the air-core coils, which are disposed to face each other.